1. Field of the Invention
The present invention relates to an automotive alternator in which electric current is supplied to a rotor coil through lead wires from a slip ring assembly.
2. Description of the Related Art
FIG. 8 is a cross section of a conventional automotive alternator, FIG. 9 is a perspective of the rotor in FIG. 8 (part of a fan has been removed), and FIG. 10 is a partial cross section of FIG. 9.
This automotive alternator includes: a case 3 consisting of an aluminum front bracket 1 and an aluminum rear bracket 2; a shaft 6 disposed in the case 3 to one end of which a pulley 4 is secured; a Lundell-type rotor 7 secured to the shaft 6; fans 5 secured to both ends of the rotor 7; a stator 8 secured to the inner wall of the case 3; a slip ring assembly 9 secured to the shaft 6 for supplying electric current to the rotor 7; a pair of brushes 10; a brush holder 11 accommodating the brushes 10; a rectifier 12 in electrical contact with the stator 8 for converting alternating current generated in the stator 8 into direct current; a heat sink 18 fitted over the brush holder 11; and a regulator 19 fastened to the heat sink 18 with adhesive for adjusting the alternating current generated in the stator 8.
The rotor 7 includes: a rotor coil 13 composed of wire wound onto a bobbin 14 for generating magnetic flux by passing electric current therethrough; and a pole core 15 disposed so as to cover the rotor coil 13 in which magnetic poles are produced by the magnetic flux generated by the rotor coil 13. The pole core 15 includes a first pole core body 21 and a second pole core body 22, each composed of magnetic poles 32 which mutually intermesh, and is prepared by a casting method using mainly low-carbon steel.
The stator 8 includes: a stator core 16; and a stator coil 17 composed of wire wound into the stator core 16 in which an alternating current is generated by changes in the magnetic flux from the rotor coil 13 as the rotor 7 rotates.
The slip ring assembly 9 includes: slip rings 40 on which the tips of the brushes 10 slide; terminals 41 electrically connected to the slip rings 40; and a resin portion 42 into which the shaft 6 is pressed, the terminals 41 being embedded by insertion molding into the resin portion 42 except for a portion thereof.
The rectifier 12 includes: an arc-shaped positive-side heat sink 24 having a plurality of fins 24a on the reverse side thereof; a plurality of positive-side diodes 23 secured by soldering to a surface of the positive-side heat sink 24; an arc-shaped negative-side heat sink 26 having a plurality of fins 26a on the reverse side thereof; a plurality of negative-side diodes 25 secured by soldering to the negative-side heat sink 26; and a circuit board 27 for electrically connecting each of the diodes 23 and 25 to the stator coil 17, the rectifier 12 converting the three-phase alternating current generated by the stator 8 into direct current.
The positive-side heat sink 24 and the negative-side heat sink 26 are composed of aluminum which has high thermal conductivity, and the radially outer negative-side heat sink 26 is grounded by direct attachment to the case 3. The positive-side diodes 23 and negative-side diodes 25 are formed by molding resin so as to have an overall rectangular shape.
Arc-shaped stays 31 are disposed at even pitch around one of the flanges 30 of the bobbin 14 of the rotor 7. These stays 31 are engaged in root portions 33 cut into arc shapes between the claw-shaped magnetic poles 32 of the first pole core body 21 to prevent relative displacement between the rotor 7 and the pole core 15 in the circumferential direction. Winding portions 34 having an E-shaped cross section are integrally disposed on a pair of opposing stays 31. Base end portions 35b of lead wires 35 leading from the rotor coil 13 are doubly wound onto these winding portions 34. These lead wires 35 lie in grooves 36 extending from the root portions 33 to the slip ring assembly 9. The lead wires 35 are engaged by hooks 37 formed into the resin portion 42, and the tips 35a of the lead wires 35 are doubly wound onto the tips of the terminals 41. Middle portions of the lead wires 35 are covered by insulation tubing 38 fastened to the grooves 36 with adhesive 44. This insulation tubing 38 is provided to prevent the corners 43 of the root portions 33 from contacting the lead wires 35 and damaging the enamel coating of the lead wires 35 by abrasion.
In a vehicle alternator of the above construction, a current is supplied from a battery (not shown) through the brushes 10 and slip rings 40 to the rotor coil 13, whereby magnetic flux is generated, giving rise to a magnetic field, and at the same time the pulley 4 is driven by the engine and the rotor 7 is rotated by the shaft 6, so that a rotating magnetic field is imparted to the stator coil 17 and electromotive force is generated in the stator coil 17. This alternating electromotive force passes through the diodes 23 and 25 of the rectifier 12 and is converted into direct current, the magnitude thereof is adjusted by the regulator 19, and the battery is recharged.
Now, because the pulley ratio between the crank pulley (not shown) of the engine and the pulley 4 of the alternator is normally between 1:2.2 and 1:2.7, the rotor 7 of the automotive alternator is subjected to high operating speeds of 15,000 rpm or more and increases and decreases in rotational load due to sudden acceleration and deceleration of the engine, depending on the rotational frequency of the engine.
Furthermore, during power generation at high speed, the first pole core body 21 and the second pole core body 22 vibrate greatly in the direction of arrow B in FIG. 13 due to magnetic attraction resulting from power generation arising in the gap A between the stator 8 and the rotor 7.
When the alternator is generating power, the rotor coil 13, the stator coil 17, the positive-side diodes 23, the negative-side diodes 25, and the regulator 19 constantly generate heat. For example, in an alternator with a rated output current in the 100 A class, the amount of heat generated is 60 W in the rotor coil 13, 500 W in the stator coil 16, a total of 120 W in the positive-side diodes 23 and the negative-side diodes 25, and 6 W in the regulator 19. The excessive generation of heat by these heat-generating bodies causes deterioration in the performance of the alternator and reduces the working life of the parts.
For that reason, the fans 5 are rotated together with the rotation of the rotor 7, external air is introduced into the case 3 through openings C in the case 3 by this rotation, and the external air flows as indicated by the arrows D in FIG. 8. Thus, after cooling the negative-side heat sink 26, the negative-side diodes 25, the positive-side heat sink 24, and the positive-side diodes 23, the external air is directed radially outwards by the fans 5, cools the end portions 17a of the stator coil 17 in the rear end, and is expelled to the outside through openings E.
External air is also introduced into the case 3 through openings F by the rotation of the fans 5, and the external air flows as indicated by the arrow G in FIG. 8. Thus, after cooling the power transistors of the regulator 18, the external air is directed radially outwards by the fans 5, cools the end portions 17a of the stator coil 17 in the rear end, and is expelled to the outside through openings H.
Similarly, external air introduced through openings I in the front bracket 1 is directed radially outwards by the fans 5, cooling the end portions 17b of the stator coil 17 in the front end. The external air is then expelled outside the case 3 through openings J.
During actual operation of a vehicle, because the ambient temperature within the engine room of the vehicle is a high 100.degree. C., instantaneous temperatures can rise to approximately 200.degree. C. in the rotor coil 13, the stator coil 17, and the diodes 23 and 25, and to approximately 150.degree. C. in the heat sink 18 of the regulator 19. Thus, the slip ring assembly 9 and the lead wires 35 are exposed to temperatures of 150.degree. C. or more due to the heat of air which has exchanged heat with the heat generating bodies such as the diodes 23 and 25, and due to radiant heat from the stator coil 16.
In the above automotive alternator, the rotor 7 is exposed to high-speed rotation, sudden acceleration and deceleration, high temperatures, and excessively large vibrations, subjecting the lead wires 35, which electrically connect the slip ring assembly 9 to the rotor coil 13, to centrifugal and vibrational forces.
In particular, because the winding portions 34, which are integrated with the stays 31, are disposed outside the outer circumferential surface of the rotor coil 13, one problem has been that the length of the lead wires 35 leading from the winding portions 34 to the slip ring assembly 9 is that much greater and the centrifugal force acting on the lead wires 35 is therefore greater due to the increase in the radial dimensions and weight of the lead wires 35, generating large stresses in the lead wires 35 and making the occurrence of breakages that much more frequent, thereby risking stoppage of the current supply required to generate the rotating magnetic field of the rotor 7 and consequently causing cessation of power generation.
Furthermore, because lead wires 35 rise up between the winding portion 34 and the corners 43 of the root portions 33, another problem has been that the lead wires 35 are susceptible to vibrations focused on the corners 43 and there has been a risk of breakages in the lead wires 35 at the corners 43, for example.